1. Field of the Invention
The invention relates to a method of producing cells for liquid crystal displays, which are known as LCD's, these cells being referred to hereinbelow as LC cells. The invention further relates to the cell produced by this method and to an apparatus for applying the method.
2. Description of the Prior Art
As is well known, most industrially produced LC cells are made with the aid of two planar cell wall elements, of which at least one is light-transmissive. The two elements, for example consisting of glass plates or thin glass sheets for the bottom plate and cover plate with transparent electrode or raster electrode layers, are joined together to form a planar cell cavity suitable for receiving the LC composition, for example by applying a suitable bonding medium (glass solder) to the edges of the plates and thereafter heating the suitably adjusted plates, so as to obtain a physically impervious and chemically resistant bond between the cell wall elements which are spaced apart in accordance with the desired thickness of the cell cavity, for example 10 .mu.m.
Since the known glass solder requires bonding temperatures on the order of 500.degree. C., distortion of the plates frequently occurs, that is to say the rate of rejection is high. Furthermore, these methods of production are not suitable when in the course of their production the cell wall elements have to be provided with boundary layers of organic material.
It has already been proposed to replace the glass solder used in the production of LC cells by organic adhesives, for example thermoplastic adhesives based on thermoplastic phenoxy resins (see German Offenlegungsschrift No. 2,443,386). The thermoplastic adhesive, in which modifiers, for example cross-linking agents, thixotropy agents, adhesion promoters, and the like are incorporated, are for example applied by screen printing to the desired adhesion points of the cell wall elements. After the elements have been joined together, the thermoplastic adhesive is melted at temperatures of about 120.degree. C. and then solidified. Other organic adhesives have also been proposed for the production of LC cells, for example polyethylene--tetrafluoroethylene copolymers having melting temperatures of about 300.degree. C. (reports of the Res. Lab., Asahi Glass Co., Ltd., volume 25, No. 2 (1975); pages 71-79), but require special pretreatment of the plates.
When organic adhesives are used, the spacing of the cell wall elements, that is to say the thickness of the cell cavity, is generally a problem, this probably being mainly caused by the difficulty of verifying changes of viscosity of organic adhesives at temperatures in the melting and hardening ranges. This problem can be solved only partially by using spacers, for example the admixing of glass pellets of defined diameter. When customary setting adhesives are used the additional difficulty arises that the curing of typical irreversibly setting compositions, that is to say compositions polymerisable irreversibly and with cross-linking, frequently occurs without a defined temperature threshold and accordingly takes place "slinkingly", which in turn can lead to undesirable inhomogeneity as the result of local variation of heat transmission.
From coating techniques numerous kinds of varnishes and printing inks are known which contain a constituent which can be irreversibly set by UV radiation and can be very rapidly or abruptly set by brief irradiation (see for example Farbe and Lack, 82, 10 (1976), pages 973-978, as well as 82, 5 (1976), pages 391-393; Modern Plastics Int., October 1976, pages 56-58; Materials Engineering, 5-76, pages 40-42 and 73-85, and also literature of the appropriate suppliers).
In connection with optical purposes, irreversibly setting systems of this kind have been proposed solely for the adhesive bonding of lenses; the use of photolacquers for the selective coating of glass plates for LC cells with a thermoplastic polymer formed on the plates by the action of light has been proposed in German Offenlegungsschrift No. 2,322,616.